This invention relates to a process and system for retorting hydrocarbon-containing material, and more particularly, to a fluid bed process and system for retorting solid, hydrocarbon-containing material such as oil shale, coal and tar sand.
Researchers have now renewed their efforts to find alternate sources of energy and hydrocarbons in view of recent rapid increases in the price of crude oil and natural gas. Much research has been focused on recovering hydrocarbons from solid hydrocarbon-containing material such as oil shale, coal and tar sand by pyrolysis or upon gasification to convert the solid hydrocarbon-containing material into more readily usable gaseous and liquid hydrocarbons.
Vast natural deposits of oil shale found in the United States and elsewhere contain appreciable quantities of organic matter known as "kerogen" which deciomposes upon pyrolysis or distillation to yield oil, gases and residual carbon. It has been estimated that an equivalent of 7 trillion barrels of oil are contained in oil shale deposits in the United States with almost sixty percent located in the rich Green River oil shale deposits of Colorado, Utah, and Wyoming. The remainder is contained in the leaner Devonian-Mississippian black shale deposits which underlie most of the eastern part of the United States.
As a result of dwindling supplies of petroleum and natural gas, extensive efforts have been directed to develop retorting processes which will economically produce shale oil on a commercial basis from these vast resources.
Generally, oil shale is a fine-grained sedimentary rock stratified in horizontal layers with a variable richness of kerogen content. Kerogen has limited solubility in ordinary solvents and therefore cannot be recovered by extraction. Upon heating oil shale to a sufficient temperature, the kerogen is thermally decomposed to liberate vapors, mist, and liquid droplets of shale oil and light hydrocarbon gases such as methane, ethane, ethene, propane and propene, as well as other products such as hydrogen, nitrogen, carbon dioxide, carbon monoxide, ammonia, steam and hydrogen sulfide. A carbon residue typically remains on the retorted shale.
Shale oil is not a naturally occurring product, but is formed by the pyrolysis of kerogen in the oil shale. Crude shale oil, sometimes referred to as "retort oil," is the liquid oil product recovered from the liberated effluent of an oil shale retort. Synthetic crude oil (syncrude) is the upgraded oil product resulting from the hydrogenation of crude shale oil.
The process of pyrolyzing the kerogen in oil shale, known as retorting, to form liberated hydrocarbons, can be done in surface retorts in aboveground vessels or in situ retorts underground. In principle, the retorting of shale and other hydrocarbon-containing materials, such as coal and tar sand, comprises heating the solid hydrocarbon-containing material to an elevated temperature and recovering the vapors and liberated effluent. However, as medium grade oil shale yields approximately 20 to 25 gallons of oil per ton of shale, the expense of materials handling is critical to the economic feasibility of a commercial operation.
In order to obtain high thermal efficiency in retorting, carbonate decomposition should be minimized. Colorado Mahogany zone oil shale contains several carbonate minerals which decompose at or near the usual temperature attained when retorting oil shale. Typically, a 28 gallon per ton oil shale will contain about 23% dolomite (a calcium/magnesium carbonate) and about 16% calcite (calcium carbonate), or about 780 pounds of mixed carbonate minerals per ton. Dolomite requires about 500 BTU per pound and calcite about 700 BTU per pound for decomposition, a requirement that would consume about 8% of the combustible matter of the shale if these minerals were allowed to decompose during retorting. Saline sodium carbonate minerals also occur in the Green River formation in certain areas and at certain stratigraphic zone. The choice of a particular retorting method must therefore take into consideration carbonate decomposition as well as raw and spent materials handling expense, product yield and process requirements.
In surface retorting, oil shale is mined from the ground, brought to the surface, crushed and placed in vessels where it is contacted with a hot heat transfer carrier, such as hot spent shale, sand or gases, or mixtures thereof, for heat transfer. The resulting high temperatures cause shale oil to be liberated from the oil shale leaving a retorted, inorganic material and carbonaceous material such as coke. The carbonaceous material can be burned by contact with oxygen at oxidation temperatures to recover heat and to form a spent oil shale relatively free of carbon. Spent oil shale which has been depleted in carbonaceous material is removed from the retort and recycled as heat carrier material or discarded. The liberated hydrocarbons and combustion gases are dedusted in cyclones, electrostatic precipitators, filters, scrubbers or pebble beds.
Some well-known processes of surface retorting are: N-T-U (Dundas Howes retort), Kiviter (Russian), Petrosix (Brazilian), Lurgi-Ruhrgas (German), Tosco II, Galoter (Russian), Paraho, Koppers-Totzek, Fushum (Manchuria), Union rock pump, gas combustion and fluid bed. Process heat requirements for surface retorting processes may be supplied either directly or indirectly.
Directed heated surface retorting processes, such as the N-T-U, Kiviter, Fusham and gas combustion processes, rely upon the combustion of fuel, such as recycled gas or residual carbon in the spent shale, with air or oxygen within the bed of shale in the retort to provide sufficient heat for retorting. Directly heated surface retorting processes usually result in lower product yields due to unavoidable combustion of some of the products and dilution of the product stream with the products of combustion. The Fusham process is shown and described at pages 101-102, in the book Oil Shales and Shale Oils, by H. S. Bell, published by D. Van Norstrand Company (1948). The other processes are shown and described in the Synthetic Fuels Data Handbook, by Cameron Engineers, Inc. (second edition, 1978).
Indirectly heated surface retorting processes, such as the Petrosix, Lurgi-Ruhrgas, Tosco II and Galoter processes, utilize a separate furnace for heating solid or gaseous heat-carrying material which is injected, while hot, into the shale in the retort to provide sufficient heat for retorting. In the Lurgi-Ruhrgas process and some other indirect heating processes, raw oil shale or tar sand and a hot heat carrier, such as spent shale or sand, are mechanically mixed and retorted in a screw conveyor. Such mechanical mixing often results in high temperature zones conducive to undesirable thermal cracking as well as causing low temperature zones which result in incomplete retorting. Furthermore, in such processes, the solids gravitate to the lower portion of the vessel, stripping the retorted shale with gas causing lower product yields due to reabsorption of a portion of the evolved hydrocarbons by the retorted solids. Generally, indirect heating surface retorting processes result in higher yields and less dilution of the retorting product than directly heated surface retorting processes, but at the expense of additional materials handling.
Surface retorting processes with a moving bed are typified by the Lurgi coal gasification process in which crushed coal is fed into the top of a moving bed gasification zone and upflowing steam endothermically reacts with the coal. A portion of the char combusts with oxygen below the gasification reaction zone to supply the required endothermic heat of reaction. Moving bed processes can be disadvantageous because the solids residence time is usually long, necessitating either a very large contacting or reaction zone or a large number of reactors. Moreover, moving bed processes often cannot tolerate excessive amounts of fines.
Surface retorting processes with entrained beds are typified by the Koppers-Totzek coal process in which coal is dried, finely pulverized and injected into a treatment zone along with steam and oxygen. The coal is rapidly partially combusted, gasified, and entrained by the hot gases. Residence time of the coal in the reaction zone is only a few seconds. Entrained bed processes are disadvantageous because they require large quantities of hot gases to rapidly heat the solids and often require the raw feed material to be finely pulverized before processing.
Fluid bed surface retorting processes are particularly advantageous. The use of fluidized-bed contacting zones has long been known in the art and has been widely used in fluid catalytic cracking of hydrocarbons. When a fluid is passed at a sufficient velocity upwardly through a contacting zone containing a bed of subdivided solids, the bed expands and the particles are buoyed and supported by the drag forces caused by the fluid passing through the interstices among the particles. The superficial vertical velocity of the fluid in the contacting zone at which the fluid begins to support the solids is known as the "minimum fluidization velocity." The velocity of the fluid at which the solid becomes entrained in the fluid is known as the "terminal velocity" or "entrainment velocity." Between the minimum fluidization velocity and the terminal velocity, the bed of solids is in a fluidized state and it exhibits the appearance and some of the characteristics of a boiling liquid. Because of the quasi-fluid or liquid-like state of the solids, there is typically a rapid overall circulation of all the solids throughout the entire bed with substantially complete mixing, as in a stirred-tank reaction system. The rapid circulation is particularly advantageous in processes in which a uniform temperature and reaction mixture is desired throughout the contacting zone.
Typifying those prior art fluidized bed retorting processes, retorting processes with various types of baffles, deflectors or downcomers, fluid catalytic cracking processes, and similar processes are the Union Carbide/Battelle coal gasification process, the fluid coker and flexicoking processes described at page 300 of the Synthetic Fuels Data Handbook, by Cameron Engineers, Inc. (second edition, 1978) and those found in U.S. Pat. Nos. 1,546,659; 1,676,675; 1,690,935; 1,706,421; 2,471,119; 2,506,307; 2,518,693; 2,542,028; 2,582,711; 2,608,526; 2,626,234; 2,675,124; 2,700,644; 2,717,869; 2,726,196; 2,757,129; 2,788,314; 2,793,104; 2,813,823; 2,901,402; 3,083,471; 3,152,245; 3,297,562; 3,318,798; 3,501,394; 3,640,849; 3,663,421; 3,803,021; 3,803,022; 3,841,992; 3,976,558; 3,980,439; 4,035,152; 4,064,018; 4,087,347; 4,125,453; 4,133,739; 4,137,053; 4,141,794; 4,148,710; 4,152,245; 4,188,184; 4,193,760; 4,210,491; 4,243,489. These prior art processes have met with varying degrees of success.
Prior art gas fluidized bed processes usually have a dense particulate phase and a bubble phase, with bubbles forming at or near the bottom of the bed. These bubbles generally grow by coalescence as they rise through the bed. Mixing and mass transfer are enhanced when the bubbles are small and evenly distributed throughout the bed. When too many bubbles coalesce so that large bubbles are formed, a surging or pounding action results, leading to less efficient heat and mass transfer.
A problem with many prior art fluidized bed processes is the long residence time at high temperatures which results in many secondary and undesirable side reactions such as thermal cracking, which usually increases the production of less desirable gaseous products and decreases the yield and quality of desirable condensable products. Therefore, in any process designed to produce the maximum yield of high quality condensable hydrocarbons, it is preferred that the volatilized hydrocarbons are quickly removed from the retorting vessel in order to minimize deleterious side reactions such as thermal cracking.
Another problem with many prior art processes, particularly with countercurrent fluidized bed flow processes, is that after the shale oil has been vaporized, it then comes in contact with countercurrent flowing solids which are at a much cooler temperature, which leads to condensation of a portion of the shale oil and reabsorption of a portion of the vaporized shale oil into the downward flowing shale. This condensation and reabsorption leads to coking, cracking and polymerization reactions, all of which are detrimental to producing the maximum yield of condensable hydrocarbons.
A further problem with many prior art fluidized bed processes is that they often have low lateral mixing and high backmixing resulting in poor plug flow, slow retorting rates and excessive bed volumes. Moreover, many prior art fluidized bed processes require excessively high fluidizing velocities and pressures. Some prior art fluidizing processes even specify heat carrier material that is larger than the crushed raw oil shale particles.
It is therefore desirable to provide an improved fluid bed process which overcomes most, if not all, of the preceding problems.